The present invention relates generally to speed controls for pneumatic devices. More particularly, the present invention relates to centrifugal governors for rotary air motors such as rotary vane air motors.
Many pneumatic tools use a rotary vane air motor as an input. Rotary vane air motors utilize a flow of compressed air to turn a shaft. In rotary vane air motors, a hub includes a plurality of vanes across which a pressure differential is produced to turn the shaft. Typically, such motors are powered by a supply of compressed that is provided at a constant pressure. As such, the speed of the motor changes only depending on the load placed on the shaft. For example, in paint agitators, the speed of an air motor used to rotate the agitator increases as the volume of paint in its container diminishes. The lack of an air motor speed control is acceptable in certain situation where precise speed control is not needed or where the operator can skillfully control performance of the pneumatic tool. It is, however, desirable to limit the speed at which the air motor can operate to prevent adverse operating conditions of the pneumatic tool. For example, it is desirable to limit the speed of paint agitators to prevent unnecessary introduction of air into the paint. Thus, many rotary vane air motors are provided with governors that simply limit the top speed of the output shaft.
Centrifugal governors are conventionally used with pneumatic tools powered by rotary vane air motors. Centrifugal governors typically comprise a pair of weights that are coupled to the air motor shaft. As the shaft rotates, the weights are forced radially outward away from the shaft under centrifugal force. A stop mechanism is positioned adjacent the weights such that air supplied to the motor is cut off when the shaft rotates fast enough to propel the weights into the stop mechanism. These types of centrifugal governors thus only provide a simple on/off mechanism. So long as the shaft rotates below a threshold speed, the air motor will rotate at whatever speed is dictated by the shaft load. At a threshold speed, the air motor is temporarily deprived of all air flow until the shaft speed slows down.
Pneumatic tools and rotary vane air motors can be provided with external speed controls that permit variation in the air motor shaft speed. For example, a restriction, such as a needle valve, can be provided at the exhaust of the air motor to limit the amount of air that can leave the motor at high speeds, thereby limiting the pressure drop across the motor to prevent speed increases. Similarly, the inlet to the motor can be throttled to achieve the same result. These throttles, however, do not control shaft speed very well as loading on the shaft changes. It is desirable to be able to more accurately control the speed of air motors independent of the load placed on the shaft without stopping rotational power to the shaft, such as for use in paint agitator applications. There is, therefore, a need for improved speed control and governor systems in pneumatic tools and air motors.